1. Field of the Invention
The present invention relates to image encoding, and more particularly to a method for intra prediction coding of image data that can improve the compression ratio of image data.
2. Description of the Related Art
In general, in the case of intra-coding macro blocks in the H.264/AVC (Advanced Video Coding), which is one of the image compression standards, coding is performed through spatial prediction using neighboring pixel values already coded. Intra modes include intra 4×4 modes and intra 16×16 modes. In accordance with the prediction direction, the intra 4×4 modes include nine modes as illustrated by FIGS. 1A-1I, and the intra 16×16 modes include four modes as illustrated by FIGS. 2A-2D.
In general, according to the intra prediction coding method, a current macro block is coded in 13 modes including the intra 4×4 modes and the intra 16×16 modes, and then the intra coding is performed in the mode having the lowest cost. More specifically, four intra 16×16 prediction modes are performed with respect to the current macro block, and the mode having the lowest cost is selected. Then, nine intra 4×4 prediction modes are performed in order with respect to 16 sub-blocks in the unit of a 4×4 sub-blocks, and then the mode having the lowest cost is selected for each sub-block. Finally, by comparing the costs of the intra 4×4 prediction modes obtained by adding the costs of the intra 16×16 prediction modes to the costs of the 16 sub-blocks, the mode having the lowest cost is finally selected.
However, in the neighborhood of the blocks positioned on a boundary region of the current frame, no pixel exists in the upper region as indicated by block a in FIG. 3, no pixel exist in the left region as indicated by block b, and no block exists in the upper and left regions as indicated by block c. Accordingly, with respect to the blocks positioned on the boundary regions of the current frame, no coding can be performed in some of the 13 modes as described above, and thus the performance of the compression of the blocks positioned on the boundary region of the current frame deteriorates.